Liquid crystal displays are commonly used as image display devices for personal communication devices such as mobile phones and personal digital assistants (PDAs). For many years now displays have been designed for multiple users and optimized so that viewers can see the same good image quality and the same image from different angles with respect to the display. This assumes that the multiple users require the same information from the display. However, there are many applications where it would be desirable to be able to see different information from the same display in spatially separated viewing windows, such as for 3D or dual view operation. For example, in computer games when each player may wish to view the game from his or her own perspective. This is currently done by each player viewing their unique perspective on individual screens, which takes up a lot of space and is not practical for portable games. By showing more than one image in more than one viewing window from the same display, there can be a considerable saving in space and cost. There is also the ability to preclude the users from seeing each other's views, which may be a desirable option in security applications such as banking or sales transactions as well as games.
Several techniques are known in the prior art for producing three-dimensional images. These techniques include: computer graphics which simulate 3D images on two-dimensional displays; stereoscopic displays where left and right retinal images are mentally fused into one image; and holographic images which reconstruct the actual wavefront structure reflected from an object.
Three dimensional displays are classified into two types depending on the method used to supply the different views to the eyes (or different viewers).
Stereoscopic displays typically display both of the images over a wide viewing area. However, each of the views is encoded, for instance by colour, polarization state or time of display, so that a filter system of glasses worn by the observer can separate the views and will only let each eye see the view that is intended for it. Without this distinction, each eye would see both views in all spatial positions.
Autostereoscopic displays require no viewing aids to be worn by the observer but rather the two views are only visible from defined regions of space. The region of space in which an image is visible across the whole of the display unit active area is termed a “viewing region”. If the observer is situated such that one of their eyes is in the viewing region for one image and the other eye is in the viewing region for the other image of the stereoscopic pair then a correct set of views will be seen and a three-dimensional image will be perceived.
“New Autostereoscopic Display System”, Ezra et al, SPIE Vol. 2409, February 1995 describes two standard LCDs mounted at 90° with beam combining optics to send the image from one LCD to one eye and the image from the second LCD to the second eye. However, this display apparatus, which uses 2 SLMs, is too large and too expensive for the mass market.
For single panel flat panel autostereoscopic displays, the formation of the viewing regions is typically due to a combination of the pixel structure of the display unit and an optical element, generically termed a parallax optic. An example of such an optic is a parallax barrier. Parallax barrier technology was first used by Ives for 3D display purposes in 1903. The parallax barrier may be an array of transmitting slit apertures on an absorbing optical element attached to the front or rear surface of a LCD device. The slit apertures are vertical and the distance between the slits is slightly less than twice the spacing between two pixels on the display for a front parallax barrier. The barrier generally lies on the surface of a polariser at a fixed distance from the pixel plane. A viewer in a defined region will be able to see with one eye alternate columns of pixels on the display and with the other eye will be able to see the intermediate columns of pixels. If the alternate and intermediate columns show separately two homologous stereoscopic images, then the viewer is able to see a stereoscopic image. Examples of 3D LCD devices with parallax barriers may be found, for example, in: G. J Woodgate, J Harrold, A. M. S Jacobs. R. R. Mosely, D. Ezra. “Flat Panel Autostereoscopic Displays-Characterisation and Enhancement”, SPIE Vol. 3957. This display has the disadvantage that it is generally 20-30% of the brightness of the base LCD panel. This is due mainly to the dark areas of the barrier and absorption in the slit. In addition, the resolution of the 3D display is half that of the base LCD; thus 2D images and especially small text become degraded by aliasing artifacts and can be difficult to read.
EP 0829744 describes a display device electronically switchable between 2D and 3D modes of operation. It suggested to use a latent parallax barrier which can be “developed” and, thus, configure 2D to 3D system. This system has the advantages of an autostereoscopic 3D mode along with a full brightness and full resolution in 2D mode. With no “developing” polariser on the display, the latent barrier is not seen as the eye does not distinguish between polarization states. If a developing polariser is placed over the display such that the polariser axis is orthogonal to the light coming from the “absorbing” part of the retarder barrier, the polarized light from the “absorbing” stripes is blocked while light from the “transmitting” stripes passes through the polariser. However, such a display device operates in 3D mode with only a half of the resolution and less than a half of the brightness of 2D mode.
D. Trayner, E. Orr “Development in autostereoscopic displays using Holographic Optical Elements”, SPIE Proc. v. 3012, pp. 167-174, 1997 describes the use of a transmission holographic optical element (HOE) for angular separation of left/right images from a transmissive LCD illuminated from the rear by an external light source. The HOE comprises two sets of horizontal (rows) stripes, and each set of stripes reconstructs a real image of a diffuse viewing window.
In the above-mentioned prior art the LCD may be positioned immediately in front of or behind the HOE, and the LCD and HOE are illuminated from the rear by an internal light source. However, because the internal light source must be at a distance from the LCD and HOE, this makes for a relatively bulky device. Furthermore; the device cannot produce simple 2D images, and cannot be used as a 2D backlit LCD and there is therefore no ability to switch between 2D and 3D operation.
G. L. Valliath, Z. A. Coleman et al. “Design of Hologram for Brightness Enhancement in Colour LCDs”. SID98 Digest; 44.5L, 1139-1142; 1998 refers to the use of a transmission hologram for brightness enhancement of a front illuminated reflective LCD. The holographic element is permanently attached to a front surface of a reflective LCD device illuminated by a distant light source positioned at 34° offset and above a display and emitting substantially collimated light. The holographic element has no optical function for the light incident from the light source, but steers and scatters the light reflected by an internal LCD reflector into a viewing zone 0°-14° from the normal axis to a display surface. The light source has to be distant to ensure uniform and substantially collimated illumination under achromatic angle (to satisfy chromatic correction of the holographic element).
U.S. Pat. No. 6,271,808 describes a stereo head-mounted display apparatus using a single grating light valve frame sequential display. This display may be used in 2D and 3D modes. The right image is directed to a right eye of the user, and the left image is directed to the left eye of the user. In a non-stereoscopic mode, both images are the same. The grating light valve display receives the light from left and right sources and sequentially sends the image to the left and right eyes.
Physical Optics Corporation demonstrated a prototype of 3D projection display. This is described at http://www.poc.com/emerging_products/3d_display/default.asp. This 3D projection display employs two projectors for projecting left and right images to a directional screen. Left and right images are angularly separated and spatially overlapped in the plane of a projection screen.
U.S. Pat. No. 5,917,562 describes an autostereoscopic display device comprising a pair of polarised light sources and an optical system for imaging the light sources into two spatially separated viewing zones. An LCD includes an array of polarisation sensitive elements, adjacent and aligned to a pixel structure of the LCD. The optical axis of a polarising element of a first type is chosen to transmit light of the polarisation of the first light source and to absorb light of the polarisation of the second light source. The optical axis of a polarising element of a second type is chosen to transmit light of the polarisation of the second light source and to absorb light of the polarisation of the first light source. Thus, only the image generated in a pixel set associated with the polarising elements of the first type is displayed in a right viewing window and only the image generated in a pixel set adjacent to the elements of the second type is displayed in the left viewing window.
To increase the functionality of direct view transmissive displays to project enlarged images on to an external projection screen, U.S. Pat. No. 6,595,648 discloses a projection display comprising a transmissive LCD with a volume reflection hologram permanently attached to its rear surface and an external front illuminator. The reflection hologram is arranged to act like a lens to form an image of the light source which is spatially displaced from the original image of the light source formed by condensing optics. This hologram has substantially no function when the transmissive display is illuminated by backlight and operates in direct view mode. The hologram functions as a reflector and an off-axis lens.
The device of the above prior art document cannot produce 3D or dual view images.
A projection display using a reflective LCD and a polarising beam splitter is disclosed in U.S. Pat. No. 6,359,719.
Japanese Patent Application JP2002-268005 discloses a portable projection display which projects the image from the display element or its intermediate image on an eyeball of the observer.